1. Field of the Invention
The present invention relates to a compound aspherical lens constituted by laminating a plastic lens to a glass lens.
2. Related Background Art
FIG. 6 shows an example of a conventional compound aspherical lens and the supporting structure therefor.
A compound aspherical lens 10 is constituted by laminating a plastic lens 12 to an aspherical lens 11 with an adhesive or the like. An ordinary spherical lens forming a photographing lens optical system must be arranged in such a manner that a plurality of lenses are positioned on the same optical axis for the purpose of reducing aberration. However, the number of the lenses can be reduced in a photographing lens optical system with the aberration reduction effect maintained by arranging the structure to employ an aspherical lens.
The thus constituted compound aspherical lens 10 is held by a lens frame 1 as shown in FIG. 6. That is, a flange-shape supporting portion la is formed on the inner surface of the cylindrical lens frame 1 so that a shoulder 11a is abutted against the supporting portion 1a. Furthermore, a threaded portion is formed in a portion of the inner surface of the lens frame 1 so that a retaining ring 2 is screwed on the threaded portion. As a result, an end portion of the retaining ring 2 is positioned in contact with an abutment portion 12a formed in the outer periphery of the plastic lens 12 so that the end portion of the retaining ring 2 presses the abutment portion 12a. That is, the compound aspherical lens 10 is secured and supported in such a manner that the outer periphery thereof is held between the retaining ring 2 and the supporting portion 1a.
However, with the thus-constituted conventional compound aspherical lens a problem arises in that the plastic lens 12 made of a soft material can be damaged or deformed because the end surface of the retaining ring 2 directly presses the outer periphery (the abutment portion 12a) of the plastic lens 12 due to the conventional structure arranged in such a manner that the glass lens 11 and the plastic lens 12 have the same diameter. If the plastic lens 12 is deformed, the width of an air layer (the distance from another lens) is disordered or the portion in which the glass lens 11 and the plastic lens 12 are joined to each other separates, causing desired optical performance to be lost. Another problem arises in that the working efficiency at the time of the assembling process deteriorates because the fixing of the retaining ring 2 must be conducted carefully for the purpose of preventing the deformation of the plastic lens 12. Furthermore, the profile irregularity (including an inclination with respect to the optical axis) of the abutment portion 12a is influenced by the profile irregularity of the surface 11b at which the glass lens 11 and the plastic lens 12 are joined to each other. Since error factors therefore increase with respect to a single lens, a satisfactory accuracy cannot be obtained. Therefore, a problem in terms of optical performance sometimes arises in that the compound aspherical lens 10 falls down with respect to the lens frame 1 when the abutment portion 12a is held by the retaining ring 2.